The present invention relates to a disc-shaped carrier body consisting of nonmagnetic material, for a recording medium which can be magnetized particularly perpendicularly (vertically) and the at least one flat side of which has a very smooth surface, to which at least one correspondingly magnetizable storage layer can be applied. Such a disc-shaped carrier body is known, for instance, from European patent application No. A-0 120 413. The invention further relates to a method for manufacturing such a carrier body.
The principle of vertical magnetization for storing information in appropriate recording media is generally known (see, for instance, "IEEE Transactions on Magnetics", vol. MAG-16, no. 1, Jaunary 1980, pages 71 to 76 or vol. MAG-20, no. 5, September 1984, pages 657 to 662 and 675 to 680). The recording media which must be provided to use this principle, which frequently is also designated as vertical magnetization, can be present, for instance, in the form of rigid magnetic storage discs.
Such a recording medium has at least one storage layer which can be magnetized accordingly, is of predetermined thickness and of a material with vertical magneto-crystalline anisotropy, where the axis of the so-called easy magnetization of this layer is aligned perpendicularly to the surface of the recording medium. Preferred as a corresponding storage material is CoCr (see, for instance, "IEEE Transactions on Magnetics", vol. MAG-14, no. 5, September 1978, pages 849 to 51). By means of special magnetic heads, the individual pieces of information can then be written as bits in successive sections following each other along a track by appropriate magnetization of the storage layer. These bits have a predetermined dimension in the longitudinal direction of the track which is also called the wavelength. As compared to the limit which is given by the demagnetization in the case of storage according to the known principle of longitudinal (horizontal) magnetization, this dimension can be substantially smaller. Thus, the information density in the special recording medium can be increased accordingly by vertical magnetization.
The disc-shaped recording media which are provided on one or both sides with the at least one magnetic storage layer must have extremely smooth (mirror-smooth) surfaces in order to prevent in this manner difficulties with the guidance of the magnetic heads over these surfaces. Particularly for vertical magnetization, the requirements as to the surface quality of the magnetic storage discs increase with the desired higher storage densities and if the guided altitude of the magnetic heads, also called flying altitude, is reduced, since the flying altitude of these heads is generally distinctly less that 1 .mu.m for this magnetizing principle. Consequently, also a sufficiently demensionally stable and strong carrier body or an intermediate layer applied to it, on which then the at least one storage layer must be deposited, must have a correspondingly small surface roughness. The carrier body or its intermediate layer is therefore generally polished to form a correspondingly smooth mirror surface.
Accordingly, in the case of the recording medium which can be gathered from the mentioned European Patent Application, its disc-shaped carrier body of nonmagnetic material such as of a special aluminum alloy is evened-out on at least one of its flat sides, for instance, by diamond facing and lapping. On the thus pretreated carrier body, an intermediate layer of a nonmagnetic very hard material such as nickel-phosphorous with a thickness of about 50 .mu.m is deposited, usually without current. This intermediate layer is subsequently precision- machined by fine-lapping and polishing processes to form a mirror surface with a surface roughness of, for instance, 0.04 .mu.m or less and a thickness of about 30 .mu.m. To this mirror surface, the storage layer of the specially magnetizable metal is subsequently applied, where its axis of easy magnetization is oriented in the direction perpendicular to the surface of the recording medium. On this so obtained stratified structure can finally be deposited a special protective layer. In manufacturing this known recording medium, the process step for polishing the intermediate layer is relatively costly and special intermediate annealing steps must be made.